The present invention provides a single-stage melt polymerization process for production of a high molecular weight polybenzimidazole from a tetraminobiphenyl (TAB) and an isophthalic acid (IPA), where the resulting polybenzimidazole exhibits an IV (inherent viscosity) which varies little over a large range of particle sizes.
Polybenzimidazoles (PBI) are polymers of high thermal stability and are resistant to oxidative or hydrolytic degradation. Polybenzimidazole polymers may be prepared by melt polymerizing an aromatic tetraamine and a diphenylester or an anhydride of an aromatic or heterocyclic dicarboxylic acid in a one or two-stage process; see, for example U.S. Pat. Nos. Re. 26,065; 3,174,947; 3,509,108; 3,551,389; 3,433,772; 3,655,632; 4,312,976; 4,431,796; 4,452,967; 4,588,808; 4,672,104, 4,717,764; 7,038,007; and 7,060,782. In particular, U.S. Pat. No. 3,551,389 discloses a two-stage process for the production of aromatic polybenzimidazoles, in which the monomers are heated at a temperature above 170° C. in a first-stage melt polymerization zone until a foamed prepolymer is formed. The foamed prepolymer is cooled, pulverized, and introduced into a second stage polymerization zone where it is heated again to yield a polybenzimidazole polymer product. Polybenzimidazoles also may be prepared from the free dicarboxylic acids or the methyl esters of such acids.
While polybenzimidazole (PBI) polymers have always had desirable properties, the cost of this material had been quite high due in part to the cost of the raw materials. Whether run as a one step or two stage process, polybenzimidazole (PBI) polymers are usually made from a reaction of a tetraaminiobiphenyl (TAB) and a diphenyl isophthalate (DPIP). DPIP is not inexpensive and it had been suggested that a lower cost starting material might be used.
Inherent viscosity (hereinafter “IV”) is the ratio of the specific viscosity of a solution of known concentration to the concentration of solute extrapolated to zero concentration, measured in units of dL/g (deciliters/gram). Inherent or intrinsic viscosity is also called the limiting viscosity number. It is directly proportional to the polymer-average molecular weight. dL/g, IV (inherent viscosity) measure, is based on the concentration of the polymer sample tested, g/100 ml or g/deciliter. The polymer is dissolved at the 0.4% (w/v) level in 100 ml of 96.5% (+/−0.5%) sulfuric acid at 80° C. After filtering, an aliquot is measured for IV using a calibrated Cannon-Fenske capillary viscometer in a 25+/−0.1 C water bath, the flow time (sec.) of the PBI polymer solution measured vs. the dissolving solvent. IV=ln (t1/t2)/c, where the natural log of the flow time of the PBI solution (t1) over the flow time of the solvent (t2) is divided by the PBI solution concentration (c).
Plugging Value (P.V.) referred to herein is a measure of solution filterability, and is determined from the filtration rate through a fine filter paper. The PBI polymer to be evaluated is dissolved in a solvent consisting of 98% dimethylacetamide and 2% lithium chloride (DMAc/LiCl) at a known concentration of 7 to 11%. The solution is prepared by weighing the desired quantity of solvent (e.g., 93 grams) into a 250 ml glass-stoppered Erlenmeyer flask, and then weighing the desired quantity of polymer (e.g., 7 grams) into the flask. The polymer is dissolved by mixing for 2 to 4 hours. The filtration apparatus consists of a Kimpak 30 ply filter with one layer of Canton Flannel in a pressure vessel. The solution after mixing is poured into the filter chamber and 14.06 kilogram force/square centimeter gauge (200 psig) of pressure from a nitrogen cylinder is applied. The weight of the solution that passes through the filter per area is recorded with respect to time. Data is continuously generated until either the filter is emptied or the filtration rate is very slow as a result of plugging, to less than one drop every five seconds. PV can be vital for solutioning and fiber spinning applications, but not necessarily for thermoplastic molding and film applications.
Calculation of the Plugging Value is as follows: The plugging value (PV) is expressed as grams of dry polymer per square centimeter of filter area that can be passed through the filter at 14.06 kilogram force/square centimeter gauge (200 psig) before blockage occurs where
            0.07      ⁢              (                  W          -          T                )                    P      .      V      .        =  Awhere 0.07=concentration of polymer solution, weight fraction PBI.                w=weight, grams, of polymer solution plus vessel        T=weight, grams, of vessel        A=area of exposed filter surface, cm2.Example 225 grams weight of vessel plus polymer solution        150 grams weight of vessel        1 cm2 area of exposed filter surface        
      P    .    V    .    =                    0.07        ⁢                  (                      225            -            150                    )                    1        =          5.25      ⁢                          ⁢      g      ⁢              /            ⁢              cm        2            
In a paper published by Eui-Won Choe in 1994, entitled “Catalysts for the Preparation of Polybenzimidazoles” it had been suggested that isophthalic acid (IPA) could be used to replace DPIP as a starting material. This paper demonstrated on a lab scale the feasibility of IPA as a substitute monomer. What is interesting is that in this work the catalyzed compositions tended to give higher IV and PV and the uncatalyzed system showed low IV and PV values compared to the catalyzed systems. In these experiments the criteria in which the lab samples were measured on or targeted were an IV>/=0.7 and a PV>0.5. Based on these numbers it was deemed that the uncatalyzed, one-step process, polymerization samples of IPA with TAB were below target PV values. Through experimentation with the catalyzed samples it was determined that reaction temperature must be at least 380° C. The range of temperatures ran 380° C. to 430° C., with the temperature of 400° C. considered optimal.